1) Field of the Invention
This invention relates to a method and an apparatus for determining the velocity of particles moving through a tube and, in particular, to a method and an apparatus for determining the velocity of blood flowing in a vessel of the eye.
2) Prior Art
As was pointed out in U.S. Pat. No. 4,142,796, issued Mar. 6, 1979, to Charles E. Riva, and elsewhere, the ability to measure the velocity of blood flowing in a single blood vessel or in a capillary bed is very useful for medical purposes. Impairment of the blood flow in the tissues of the ocular fundus, or the retina of the eye, is associated with a large number of diseases that can lead to grave visual disorders.
The feasibility of using laser Doppler velocimetry to measure blood flow in individual retinal vessels was demonstrated in 1972 by Riva et al. (C. E. Riva, B. Ross and G. B. Benedek, Investigative Ophthalmology, Vol. 11, pps. 936 et seq., November 1972). Riva et al. measured the Doppler-shift frequency spectrum of laser light scattered from red blood cells flowing in a retinal artery of an anesthetized rabbit. The maximum Doppler frequency shift f.sub.max arising from the light scattered by the red blood cells flowing at the maximum velocity V.sub.max was estimated from the spectrum of the scattered light. V.sub.max was calculated from f.sub.max and from estimates of the intraocular scattering geometry using the general relation: ##EQU1## where .lambda. is the wavelength in vacuo of the incident laser light, n is the refractive index of the flowing medium, .theta..sub.i is the intraocular angle between the incident beam and the blood flow direction, and .theta..sub.s is the intraocular angle between the collected scattered light beams and the blood flow direction. It was assumed that the incident laser beam was perpendicular to the blood flow direction.
It has also been shown (G. T. Feke and C. E. Riva, J. Opt. Soc. Am. Vol. 68, pps. 526 et seq., 1978) that f.sub.max can be determined from Doppler-shift frequency spectra obtained from human retinal vessels using short measurement times. Furthermore, it has been shown that V.sub.max can be determined by a procedure involving the collecting of light scattered by red blood cells in two distinct directions, separated by a known angle (C. E. Riva, G. T. Feke, B. Eberli and V. Bernary, Applied Optics, Volume 18, pps. 2301 et seq., Jul. 1, 1979). It has been determined that analysis of the collected light yields an absolute measurement of V.sub.max that is independent of the exact orientation of the incident and scattered light beams with respect to the blood flow direction.
In U.S. Pat. No. 4,402,601, issued on Sep. 6, 1983, to Charles E. Riva, bidirectional LDV was performed using an apparatus, the basic component of which was a standard retinal camera. The need for a contact lens was eliminated, and the laser beam was delivered to the eye through the fundus illumination optical system of the camera. The device taught by Riva in U.S. Pat. No. 4,402,601 greatly simplified the technique of retinal blood flow measurement.
In the device taught by Riva, bidirectional laser Doppler velocimetry was used to permit absolute measurements of the velocity of red blood cells flowing through retinal vessels. In this technique the Doppler-shift frequency spectra of laser light scattered from the red blood cells were recorded for two directions of the scattered light, while the direction of the incident beam remained constant. The Doppler-shift frequency spectra, when obtained in short measurement times, exhibited large fluctuations in spectral power up to a cutoff at a frequency f.sub.max that arose from light scattered by red blood cells flowing at the maximum velocity V.sub.max at the center of the blood vessel. V.sub.max was obtained from the Doppler-shift frequency spectra using the relation: ##EQU2## wherein .lambda. was the wavelength of the incident laser beam used to perform the measurement, .DELTA.f=f.sub.2 max -f.sub.1 max was the difference between the cutoff frequencies obtained from Doppler-shift frequency spectra recorded in two directions, K and K.sub.2, n was the index of refraction of the flowing medium, a was the angle between vectors K.sub.1 and K.sub.2 and .beta. was the angle between the vector V.sub.max and its projection on the plane defined by the vectors K.sub.1 and K.sub.2.
The first absolute measurements of V.sub.max were obtained by Riva using a standard slitlamp microscope in conjunction with a low-vacuum corneal contact lens. The use of a contact lens considerably simplified the determination of the scattering geometry because the lens eliminated the corneal refraction of the Doppler shifted light. Several problems arose, however, when the technique was applied to human subjects: (a) there was a risk of corneal abrasion and infect (b) there was poor motion stabilization of the target retina because the fellow (non-target) eye was used for target fixation; (c) changes in intraocular pressure caused by application of the contact lens could affect retinal blood flow; and (d) the slitlamp instrument did not allow Doppler-shift frequency spectra to be simultaneously recorded for two directions of the scattered light or the determination of V.sub.max in vertical vessels.
While the method taught by Riva provided reliable velocity measurements, it suffered from inherent drawbacks with regard to alignment and depth resolution. Regarding alignment in the Riva method, the procedure for measuring blood velocity in a vessel consisted of focusing a laser beam on the vessel. Two beams of the light scattered by the moving particles were optically selected and focused in a plane by a lens. The two scattered light beams were collected by two optical fibers which transferred them to two photodetectors. The input apertures of the optical fibers transmitting the scattered light to the photo-detectors were moved to the two focused beams using x-y microdrives. This procedure was time consuming, particularly when the incident beam was moved to a number of different vessels, as was the case when several retinal or conjunctival vessels were measured.
The depth resolution of these measurements was determined by the optics of the eye and the optical systems used to illuminate the vessel and to detect the scattered light. In a fundus camera arrangement, for example, the resolution was insufficient to separate light scattered by the different layers of the fundus so that measurements from retinal vessels could be affected by light scattered from red blood cells moving in the choriocapillaries. This was the case, in particular, when the velocity of the moving red blood cells was measured in small retinal blood vessels.
Therefore, an object of the optical system of the present invention is to overcome the inherent drawbacks with regard to alignment and depth resolution, as well as other drawbacks in the prior art, by providing a method and apparatus for self-alignment when measuring the velocity of moving particles such as red blood cells in individual blood vessels combining the principles of confocal imaging and bidirectional laser velocimetry and incorporating the apparatus into a device that allows viewing of the blood vessel and the surrounding media.
More particularly, it is an object of the optical system of the present invention to measure the velocity of moving particles such as red blood cells in individual blood vessels of a vascular bed such as the conjunctiva and the iris in the anterior part of the eye or in the retina in the fundus of the eye.
Another object of the optical system of the present invention is to provide an instrument that is more compact and easier to use than prior art velocimetry instruments.
Another object of the optical system of the present invention is to provide better spatial definition than the spatial definition available in prior art velocimetry instruments.
Another object of the optical system of the present invention is to provide an instrument having substantially common path propagation of the illumination beam and the scattered detector beams.
Another object of the optical system of the present invention is to provide adaptability to existing ophthalmic instruments such as fundus cameras, slitlamps and laser scanning ophthalmoscopes with minimal modification of the existing instruments.
Briefly the present invention is a method and apparatus for measuring the speed of particles moving in the same direction, such as red blood cells moving in individual blood vessels, combining the principles of confocal imaging and bidirectional laser Doppler velocimetry. In the method of the invention the moving particles are illuminated by a laser beam and the light scattered by the moving particles is detected along two directions by two photodetectors. The optical systems for illuminating the moving particles and for detecting the scattered light are arranged in a confocal mode. The scattered light contains components corresponding to Doppler-shifted light from the moving particles and components corresponding to the unshifted light corresponding to the light scattered by nonmoving structures (reference beam). The shifted and unshifted components are caused to interfere at the surface of each photodetector. Electric signals from the photodetectors thus contain a spectrum corresponding to the optical spectrum of the scattered light shifted down to lower frequencies by an amount equal to the frequency of the incident laser light. The electrical signals are filtered, amplified and digitized. A computer algorithm removes the noise component from the digitized signals to determine the noise free spectrum. The maximum frequency shift, corresponding to the maximum velocity of the particles (centerline velocity), is then determined from the noise free spectrum. In the bidirectional mode, the signals from the two detectors are analyzed to obtain two maximum frequency shifts. From the two maximum frequency shifts, the frequency of the incident laser beam, the angle between both scattered beams, and the maximum velocity of the moving particles are obtained.